tools/gdbinit: check job is not NULL before accessing internal datas

tools/gdbinit

@@ -20,13 +20,15 @@ define starpu-print-job
   set $job = (struct _starpu_job *)$arg0
 
   printf "StarPU Job (%p)\n", $job
-  printf "\ttask:\t\t\t\t<%p>\n", $job->task
-  printf "\tsubmitted:\t\t\t<%d>\n", $job->submitted
-  printf "\tterminated:\t\t\t<%d>\n", $job->terminated
-  printf "\tjob_id:\t\t\t\t<%d>\n", $job->job_id
-  if _starpu_use_fxt == 1
-    if $job->model_name
-      printf "\tmodel_name:\t\t\t<%s>\n", $job->model_name
+  if ($job != NULL)
+    printf "\ttask:\t\t\t\t<%p>\n", $job->task
+    printf "\tsubmitted:\t\t\t<%d>\n", $job->submitted
+    printf "\tterminated:\t\t\t<%d>\n", $job->terminated
+    printf "\tjob_id:\t\t\t\t<%d>\n", $job->job_id
+    if _starpu_use_fxt == 1
+      if $job->model_name
+        printf "\tmodel_name:\t\t\t<%s>\n", $job->model_name
+      end
     end
   end
 end